Processes for removing water from industrial gases or from a mixture of gaseous or liquid hydrocarbons by passing the latter through adsorbents are known.
Molecular sieves are particularly employed for such processes. However, molecular sieves deteriorate rapidly because of the presence of so-called insoluble, that is to say liquid, water in the gases or liquids to be dried. In fact, this results in a loss of water removal efficiency and accelerated aging of the sieves.
To avoid this, it is known to protect molecular sieves with a layer of alumina, silica gel or active carbon, the purpose of which is to adsorb the liquid water. Combinations of alumina layers and of molecular sieves make it possible to extend the lifetime of the sieve and to ensure good drying of the gases or liquids.
However, there is still a perceptible need for an increase in the lifetime of these molecular sieves and of their adsorption effectiveness.
The aim of the invention is to propose a process for the removal of water from industrial gases or from a mixture of gaseous or liquid hydrocarbons with the aid of an adsorbent composed of the combination of layers of molecular sieves and of alumina, in order to obtain a satisfactory water removal efficiency while considerably reducing the aging of the adsorbent.
The objective of the invention is to propose a process for drying a gaseous or liquid mixture by passing the said mixture into an adsorber, characterized in that the water adsorption equilibrium zone of the adsorber comprises an upstream alumina zone and a downstream molecular sieve zone.
Other characteristics of the invention will appear on reading the description and the nonlimiting examples.
The invention relates first of all to a process for drying a gaseous or liquid mixture by passing the said mixture into an adsorber, characterized in that the water adsorption equilibrium zone of the adsorber comprises an upstream alumina zone and a downstream molecular sieve zone.
More precisely, this process deals with the removal from gaseous or liquid mixtures. The composition of this mixture may be very varied. Industrial gases or mixtures of gaseous or liquid hydrocarbons may be especially mentioned. Gases to be dried may be, for example, natural gases of methane, ethane type, etc., or associated gases obtained during the liquid/gas separation of petroleum fractions, gases originating from steam cracking or from fluid catalytic cracking (FCC), industrial gases such as pure gases: N2, O2Ar, etc., coke plant gases or gases originating from processes such as catalytic reforming, hydrocracking etc. The liquids to be dried may be natural-gas condensates of CO2 type, liquid air or LPGs (liquid petroleum gases) such as, for example, propane or butane.
These mixtures may be unsaturated, saturated or supersaturated with water.
The process uses the principle of separation of water from gases or liquids containing it by adsorption thereof on layers of aluminas and of is molecular sieves. The process consists in introducing the gases or liquids to be dried into an adsorber where they will encounter in succession a layer of alumina and a layer of molecular sieve. The introduction of the gases or liquids can take place equally well via the top as via the bottom of the adsorber, as long as the gases or liquids pass firstly into the alumina layer and only subsequently into the molecular sieve layer.
In the use of the process the alumina adsorbs water in gaseous form, but also essentially water in liquid form, whereas the molecular sieve now adsorbs only the gaseous water.
In the process according to the invention it is essential that the alumina and the molecular sieve should be introduced in precise conditions. It is necessary, firstly, that the alumina should not be situated in the water adsorption mass transfer zone of the adsorber.
The concept of mass transfer zone, employed here, corresponds to the conventional concept of mass transfer zone in the technical field of adsorptions on beds, that is to say that it is defined as the portion of the adsorber in which the concentration of the adsorbate, in this case water, on the adsorbent varies from zero concentration to the maximum adsorption concentration at the equilibrium. This mass transfer zone is generally of constant length and moves during the adsorption cycle, right from the entry of the adsorber up to its exit (in the direction of introduction of the gases or liquids).
Very precisely, the mass transfer zone is defined in this invention as the mass transfer zone taken at the instant when the water breaks through at the exit of the adsorber, that is to say at the instant when the adsorption capacity of the adsorbent is saturated and the cycle ended.
Secondly, it is necessary that the alumina should be placed in the so-called upstream zone of the water absorption equilibrium zone of the adsorber and the molecular sieve in the so-called downstream zone of the said equilibrium zone of the adsorber.
The upstream zone, comprising alumina, corresponds to the part of the equilibrium zone which is the first one in contact with the gases or liquids to be dried, and the downstream zone, comprising the molecular sieve, corresponds to the part of the equilibrium zone which is the last one in contact with the gaseous or liquid mixture to be dried.
The equilibrium zone itself also corresponds to the conventional concept of an equilibrium zone in the technical field of adsorptions on beds. It is therefore defined as the portion of the adsorber in which the concentration of the adsorbate, in this case water, on the same single adsorbent is constant. The length of this equilibrium zone increases during the adsorption cycle.
In the context of the invention the equilibrium zone is also defined as the equilibrium zone taken at the instant when the water breaks through at the exit of the adsorber, that is to say at the instant when the adsorption capacity of the adsorbent is saturated and the cycle finished, and therefore at the same instant when the mass transfer zone is defined.
In practice, the mass transfer zone is defined from adsorption isotherms of the adsorbents employed, at a pressure and temperature which are fixed, and from the internal and external diffusion coefficients of the adsorbates in the adsorbents; the equilibrium zone is determined from the adsorption isotherms of the adsorbents employed at fixed temperature and pressure.
In the equilibrium zone of the adsorber the ratio of the volume of alumina to that of the alumina and of the molecular sieve (Q) is generally not more than 0.95.
In this case and throughout the description the volumes of alumina and of molecular sieve correspond to the volumes determined at the time of charging of the adsorber.
In the equilibrium zone of the adsorber the ratio of the volume of alumina to that of the alumina and of the molecular sieve (Q) is preferably between 0.05 and 0.95; still more preferably this ratio is between 0.05 and 0.8 and more preferably between 0.2 and 0.8, as a function of the embodiments.
In the case where the gaseous or liquid mixture is supersaturated or saturated with water and according to a first embodiment, the ratio of the volume of alumina to that of alumina and of the molecular sieve in the water adsorption equilibrium zone (Q) is preferably between 0.5 and 0.8. Such a ratio of volumes will allow the water to be removed efficiently. It is considered that a gaseous mixture is supersaturated with water when it contains, at a given temperature and pressure, water in gaseous phase and in liquid phase; and that a liquid mixture is supersaturated with water when it contains miscible water and water which is not miscible with the constituents of the liquid.
According to a second embodiment, which corresponds to the case where the gaseous or liquid mixture is undersaturated or saturated with water, the ratio of the volume of alumina to that of alumina and of molecular sieve in the water adsorption equilibrium zone (Q) is preferably between 0.05 and 0.30. A gaseous mixture is undersaturated with water at a given temperature and pressure when it contains a quantity of gaseous water which is lower than that corresponding to saturation, the said saturation corresponding to the maximum solubility of water in the gaseous mixture at a given temperature and pressure. A liquid mixture is undersaturated with water at a given temperature when it contains a quantity of miscible water which is lower than that at saturation, the saturation corresponding to the maximum solubility of water in the liquid mixture at a given temperature.
The aluminas employed in the process according to the invention are generally the product of rapid dehydration of at least one aluminium hydroxide such as bayerite, hydrargillite or gibbsite, nordstrandite and/or of at least one aluminium oxyhydroxide such as boehmite, pseudoboehmite and diaspore. They may be the product in particular of the process described in Patent FR 1 108 011. It is also possible to employ alumina gels.
The special aluminas Spheralite(copyright) 501 A and C, 505 A and CR, marketed by Procatalyse are suitable for this type of process.
All types of molecular sieves which are suitable for drying gaseous or liquid mixtures can be employed. These sieves may in particular include zeolites. Zeolites chosen from the group including zeolites A and X will be used preferably. Still more preferably, molecular sieves will be employed which are composed of zeolite formed with the aid of a binder which may be a clay (kaolinite, bentonite, montmorillonite, attapulgite etc.), an alumina (alumina gel or alumina produced by the rapid dehydration of aluminium hydroxides or oxyhydroxides), an amorphous mixture of silica and alumina, a silica gel or titanium oxide.
The nature of the layer of molecular sieve and the size of the molecular sieves in the mass transfer zone and in the equilibrium zone may be different. For example, if the sieve is in the form of extrudates, their diameters may vary, depending on whether they are in the mass transfer zone or in the water adsorption equilibrium zone.
The process requires the use of at least one adsorber which is filled in the proportions and with the products defined above. The unit making use of the process may consist of a number of adsorbers operating in series or in parallel, some operating in an adsorption mode while the others operate in a regeneration mode.
The invention also relates to a process for drying a liquid or gaseous mixture as defined above, in which the mixture additionally includes HCl, and characterized in that the water adsorption zone of the adsorber is preceded, in the direction of flow of the mixture in the adsorber, by an alumina-based zone making it possible to remove the HCl present in the mixture.
The drying process according to the invention may therefore be combined more generally with processes for removal of compounds other than water, such as, for example, HCl. In this case the gaseous or liquid mixture from which it is desired to remove water and HCl is introduced into an adsorber including an alumina-based first adsorption zone for HCl and a second adsorption zone for water, which makes use of the process according to the invention, that is to say a water adsorption zone in which the equilibrium zone comprises an upstream alumina zone and a downstream molecular sieve zone.
The first adsorption zone for HCl generally consists of an alumina of the same type as those mentioned above and preferably doped with an alkaline-earth material.
The invention also relates to a process for drying a liquid or gaseous mixture as defined above, in which the mixture additionally includes H2S, and characterized in that the water adsorption zone of the adsorber is followed, in the direction of flow of the mixture in the adsorber, by a molecular sieve-based zone making it possible to remove the H2S present in the mixture.
In this case the gaseous or liquid mixture from which it is desired to remove water and H2S is introduced into an adsorber including a first adsorption zone for water, which makes use of the process according to the invention, that is to say a water adsorption zone in which the equilibrium zone comprises an upstream alumina zone and a downstream molecular sieve zone, and a molecular sieve-based second adsorption zone for H2S.
The second adsorption zone for H2S is generally based on molecular sieves of the type of those mentioned above, involving preferably type 5A molecular sieves.
Finally, the invention relates to a process for drying a liquid or gaseous mixture as defined above, in which the mixture additionally includes HCl and H2S, and characterized in that the water adsorption zone of the adsorber is preceded, in the direction of flow of the mixture within the adsorber, by an alumina-based zone making it possible to remove the HCl present in the mixture, and followed by a molecular sieve-based zone making it possible to remove the H2S present in the mixture.
Some first principles of the running conditions of the adsorbers are given below by way of examples, it being understood that a person skilled in the art will, if necessary, adapt these conditions as a function of the type of gaseous or liquid mixture being treated.
The surface velocity of the gaseous mixture in the adsorber is between 1 and 20 m/min and that of the liquid mixture between 0.1 and 2 m/min.
The pressure within the adsorber is between 600 mm Hg and 150 bars.
The temperature within the adsorber is between xe2x88x9240 and 100xc2x0 C.
When the adsorption is finished the adsorber is regenerated by introduction of a fluid while the temperature is raised and/or by reduction in the partial pressure. The temperature may be between 0 and 350xc2x0 C. and the regeneration pressure (PG) between 1 and 120 bars. In the case of reduction in the partial pressure, the pressure reduction is at most 4 bars/min.
The following examples illustrate the invention without, however, limiting its scope.